NONEQUILIBRIUM SOLUBILITY AND SEGREGATION IN ION IMPLANTED, LASER ANNEALED SILICON

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NONEQUILIBRIUM SOLUBILITY AND SEGREGATION IN ION IMPLANTED, LASER

ANNEALED SILICON

Shelby Wilson, C. White, F. Young, Jr, B. Appleton, J. Narayan

To cite this version:

Shelby Wilson, C. White, F. Young, Jr, B. Appleton, et al.. NONEQUILIBRIUM SOLUBILITY

AND SEGREGATION IN ION IMPLANTED, LASER ANNEALED SILICON. Journal de Physique

Colloques, 1980, 41 (C4), pp.C4-91-C4-96. �10.1051/jphyscol:1980416�. �jpa-00219931�

NONEQUILIBRIUM S O L U B I L I T Y AND SEGREGATION I N ION IMPLANTED, LASER ANNEALED S I L I C O N ( " )

S,R. ~ilson(+), C.W. White, F.W. Young, Jr., B.R. Appleton and J. Harayan

S o l i d S t a t e D i v i s i o n , Oak R i d g e N a t i o n a l L a b o r a t o r y Oak R i d g e , TN 37830.

Abstract.- Ion scattering and channelling measurements show that the rapid liquid phase epitaxial recrystallization induced by pulsed laser annealing incorporates ion implanted As, Sb, Ga, In and Bi into substitutional lattice sites up to maximum concentrations, Cgax, which greatly exceed equilibrium solid solubility limits.

Model calculations require a distribution coefficient from the liquid far greater than the equilibrium value to explain the measured concentration profiles. When the total dopant concentration considerably exceeds Cmax, a cell structure is formed in the near surface region, presumably as a result oSconstitutiona1 supercooling at the liquid solid interface during solidification.

Recently, it has been demonstrated that ion implanted semiconductors can be comletely annealed using radiation from Q- switched ruby or Nd-Yag lasers (pulse du- ration time 15-100 x 10-~s)jl/. During pulsed laser annealing the laser light is absorbed in the near surface region and causes the first few thousand angstroms of crystal to melt /1-5/. If the melt depth is greater than that of the damage distribu- tion, the melted region regrows epitaxial- ly with the same perfection as the substra- te crystal, and with the dopant occupying substitutional lattice sites. Calculations predict that regrowth velocities of several neters/secondes are achieved in this process

/2,4/. Furthermore, the regrown regions exhibit materials properties which cannot be achieved by conventional thermal annea- ling /1,6/! There have been several reports that the concentrations of implanted dopants occupying substitutional lattice sites in Si after pulsed laser annealing exceed conventional limits of solid solubility /1,7,8/. Since most of these systems exhi- bit retrograde solubility, this constitutes direct experimental evidence for the forma- tion of supersaturated alloys by solute trapping as discussed by Baker and Cahn/9/..

(*)Research sponsored by the Division of Materials Science, U.S. Department of Energy under contrac-t V7~7 405-eng-2 6 with Union Carbide Corporation.

(+)ORAU braduate Fellow from North Texas State University, Denton, TX 76203; Pre- sent address: Motorola Inc., Phoenix, Arizona.

This has been recently confirned for Ga in Si/8/. Also, impurities such as Cu and Fe show pronounced segregation to the surface during pulsed laser annealing /1,6,10,11/.

Segregation has been correlated with a low equilibrium distribution coefficient from the liquid defined as ko = s, C where Cs and CL are equilibrium dopant concentrations in CL the solid and the liquid phase as determined from an equilibrium phase diagran.

Significantquestions remain however concerning the materials interactions lea- ding to these metastable substitutional alloy systems, In this paper, we summarize results of a systematic study /12/ in which silicon crystals were implanted with a ran- ge of Group 111 (Ga, In) and Group V (As, Sb, and Bi) dopants and subsequently laser annealed. We find that in each of these retrograde systems, the dopant concentration in substitutional lattice sites can

exceed the conventional limits of solid solubility. Using model calculations we show that the measured dopant profiles after la- ser annealing can be explained by normal diffusion in the liquid phase and by a mo- dified interfacial distribution coefficient k l ( k l = CH/CL, where CH and Ci are the do- pant concentrations in the solid and liquid phases at the rapidly moving interface) con- siderably greater than the equilibrium value /12/ ko. Also, we find there is a maximum dopant concentration which can be incorpora- ted into substitutional lattice sites

( c ? ) , and these measured limiting

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980416

C4-92

concentrations exceed equilibrium solubili- ty limits /13/ by factors of up to %500, At higher dopant concentrations we find that a cell structure is developed in the near surface region as a result of laser annea- ling /14/. The cell structure we interpret as resulting from constitutional supercoo- ling at the liquid-solid interface during regrowth.

Silicon single crystals

(100) orien- tation) were implanted to doses in the range 1 0 ~ ~ - 1 0 ' ~ / c m ~ , at energies in the range 100-250 keV, and annealed using one pulse of radiation from a Q-switched ruby laser

(pulse duration s16

lo-' s, energy density 1,5 ~/crn~). Crystals were examined in the implanted and laser-annealed conditions using 2.5 MeV ~ e + Rutherford backscattering and ionchannelling techniques. The lattice location and dopant concentration in subs- titutional lattice sites as a function of depth were determined from aligned axial channellingspectra and from detailed angular

scans across the [110] and [lll] axial di- rections, Total dopant concentration profi- les were obtained from backscattering spec- tra obtained while continuously rotating the crystal to average over all crystallo- graphic directions.

F2gure 1 shows that dopant concentra- tion profiles in the implanted and laser 'annealed conditions for 2 0 9 ~ i and 1151,

implants, In both cases pulsed laser annea- ling has caused a considerable redistribu-

tion of the implanted dopants and segrega- tion of these dopants to the surface (%20%

in the case of Bi and %65% in the case of In),. However, ion channellingmeasurements show that the nonsegregated dopants are highly s'ubstitutional in the lattice after laser annealing (*97% for Bi and %89% for In) even though the dopant concentration greatly exceeds the conventional solid so- lubility limit /13/ (by a factor of Q100 for these two exhples).

The solid lines shown in figure 1 are calculations based on a model which incor- porates both diffusion of the dopant in the liquid, and a distribution coefficient k' from the liquid phase ,612/. Calculations /15/ of the melt front position as a

function of time predict that the melted region extends to a maximum depth of Q3200

0

A in a time of *30

lo-' s after the start of the laser pulse, The melted region then begins to solidify, proceeding toward the surface with a velocity, v, of s450 cm/s.

The dopant profile in the liquid as a func- tion of space and time is calculated by nu- merical solution of the mass diffusion equa- tion expressed in finite differences, using literature values /16/ for liquid phase diffusion coefficients.. During solidifica-

0

tion of a depth interval Ax (%25-40 A), an j

0 SOLID S O L W L W Y LIMIT SOLID SOLUBILITY LIMIT

--.

'\

DEPTH ( p m ) DEPTH lpm)

209~i(250 kbV 1.2 X 1 0 ~ ~ / c r n ~ ) ""ln (425 keV 4.2 X

40~~/crn')

in (l00)sl in

(100)si

Figure 1.: Dopant concentration profiles in sili- con compared with calculations. Horizontal lines designate equilibrium solubility limits. a)209~i

(250 keV, 1.2

10~~/cm~); b)1151n (125 KeV, 1.2

1015/cm2).

amount of dopant CA = k'CL is incorporated into the solid and the remainder, (1-k1)C;, is rejected into the liquid in a depth in- terval AX^-^. Rejected dopant is then allo- wed to diffuse in the liquid for a time At = Ax/v until an exponential profile develops in front of the interface. The calculation is repeated and continued until

0

the interface progresses to within 200 A of

the surface, where dopant remaining in the

liquid is considered to be segregated to

the surface, For these calculations, k' is

treated as a fitting parameter and best fit

values are determined from comparison of

calculations to the measured profiles using

least squares fit. In this model, we assume

that mass transport -in the liquid is by

diffusion only, k 1 is independent of dopant

concentration, regrowth velocity is constant

The solid lines in figure 1 indicate to be substantially larger than ko. Calcu- the fits to the measured profiles obtained lated values for the segregated component

0

from the nodel calculationsfor k t = 0.4 for (dopant remaining in the 200 A liquid layer) Bi and k' = 0.15 for In (see Table I). Cal- agree with measurements to within 10%. For culated profiles obtained using the equili- In, the large fraction segregated to the

Table I, Gouparison of Distribution Coefficients and Substitutional Solid Solubilities for Equilibrium and Liquid Phase Epitaxial Regrowth Conditions.

(b)~aximum in retrograde solubility (Ref.

/13/).

surface apparently is related to the signi- ficantly higher diffusion coefficient in liquid silicon. Profiles after laser annea- ling have been calculated for As, Sb, and Ga as well. Best fit values for k' are lis- ted in table I and these values are signi- ficantly larger than the equilibrium values ko.

The large values for k' relative to k indicate there is a considerable depar-

0

ture from local equilibrium at the interfa- ce during regrowth. This departure from local equilibrium is brought about by the high velocity of the liquid solid interface.

To our knowledge, these values for k' are the first determinations of nonequilibriun interfacial distribution coefficientsduring rapid solidification.

As the implanted dose was increased, the dopant concentration in substitutional lattice sites as a function of depth was found to increase until a maximum substitu- tional concentration ( c ? ) was reached above which dopants did not occupy

substitutional lattice sites, This limiting concentration was determined experimentally by using ionchannelling techniques to compa- re the total dopant concentration with the substitutional dopant concentration. Expe- rimental results for the case of high dose implants of Ga and In in silicon (after laser annealing) are shown in figures 2 and 3 where we compare the total dopant concen- tration with the substitutional dopant concentration as a function of depth.

Figure 2 shows that Ga is 96% substitutio- nal up to a maximum substitutional concen- tration (CY) ^{of %4..5}

10'~/cm~ while figure 3 shows that In %65% substitutional up to a value for CY ^{of $1.5}

10~~/crn~.

(The substitutional fraction for In in fi- gure 3 in the deep tail region is probably lower than the real value due to dechannel-

0

ling in the first 1000 A of the crystal).

The values for ? C for each of the dopants we have studied are listed in table I and compared with corresponding equilibrium solid solubility limits (cO).

S

0%-94

I I

I

I

o TOTAL Ga SUBSTITUTIONAL Ga

- -

-

- _,."

-

- -

-

-

- - -

- -

- -

- -

- -

- ^-

- ^-

- ^-

SOLID SOLUBILITY

'9

LIMIT

8

- -

- ^-

I

I

I

10'9

0 0. I 0.2

F i g u r e 2

Comparison of t o t a l a n d s u b s t i t u t i o n a l dopant c o n c e n t r a t i o n f o r 6 9 ~ a (100 keV, 6.4

1015/cm2) i n s i l i c o n a f t e r l a s e r annealing.

0.3

In every case CY significantly exceeds

c :

, by factors that range from 4 in the case of As to 500 in the case of Bi. Each of these dopanks exhibits retrograde solubili- ty /9/ in that the solidus line on the

DEPTH ( p m )

6 9 ~ a ( 1 0 0 keV, 6.44 x 1 0 ' ~ / c m ~ ) in (100) Si After Laser Annealing

o TOTAL In SUBSTITUTIONAL I n

equilibrium phase diagram passes through a maximum at temperatures which has no appa- rent relation to any eutectic temperature /13/. As pointed out by others /9/, if a composition in solid solution is measured that exceeds the retrograde maximum, this indicates that a positive departure from local equilibrium must have occurred at the liquid-solid interface during regrowth. The large values for cmax relative to : C for

S

each of these dopants therefore clearly demonstrates a strong departure from local equilibrium associated with the dopant in- corporation. Incorporation of dopants into substitutional lattice sites at these high grrowth velocities presumably takes place by "solute trapping" as discussed else- where /9/.

The very high values for k' and

compared to corresponding equilibriumvalues show that the nonequilibrium phase diagram appropriate to our laser annealing condi- tions is significantly different from the equilibrium phase diagram, These results indicate that the solidus line shifts to higher concentrations, and is displaced toward the liquidus line.. (Note that no nucleation of the silicon phase is necessa- ry since this is epitaxial regrowth; hence we expect that the liquidus line would not be greatly changed by the very rapidgrowth.) The fact that a limiting substitutional concentration is reached shows that the nonequilibrium solidus line also passes through a maximum concentration, which is considerably higher than the equilibrium maximum, The values for k' and CY ^should

be dependent on the regrowth velocity, and experiments are in progress to determine this dependence.. An upper limit to the growth velocity that can be used to achieve epitaxial regrowth is probably set by the time required for bond rearrangement to

F i g u r e 3

Comparison of t o t a l and s u b s t i t u t i o n a l region. For these high dose cases, we find

d 0 ~ a n t c o n c e n t r a t i o n f o r 1151n (125

that the non substitutional dopant is

. .

1016/cm2) i n s i l i c o n a f t e r l a s e r annealing.

take place at the liquid-solid interface.

4oq9 I

I

0 0.f 0.2

The dopant concentration which is

DEPTH (pm)

non substitutional in the lattice after

4151~ ( f 2 5 keV, 1.25% 10'6/cm2) in (100) ~i After ~ a s e r

laser annealing (Figs. 2 and 3 ) is not

Annealing

randomly distributed in the near surface

annealing /14/. The bright field TEI: micro- graph of figure 4a shows the near surface cell structure in the high dose In implan- ted crystal. (The total and substitutional dopant concentrations as a function of depth for this crystal are given in figure 3.) The interior of each cell in figure 4a is a defect free column of epitaxial silicon with an In concentration of %1.5

1o2'/cm3 trapped in substitutional lattices sites.

F i g u r e 4

E l e c t r o n microscopy of 1151n (125 keV, 1.25

1016/cm2) i n s i l i c o n a f t e r l a s e r annealing, a ) B r i g h t f i e l d microgri~ph showing t h e c e l l s t r u c t u r e developed i n t h e n e a r s u r f a c e region.

l e c t e d a r e a d i f f r a c t i o n p a t t e r n showing 110 I n s p o t s n e a r missing 200 s i l i c o n s p o t s . c ) Dark f i e l d micrograph o b t a i n e d u s i n g 110 I n s p o t s .

The cell walls in figure 4a however contain massive concentrations of segregated In,

0

and the walls extend to a depth of %lo00 A into the crystal. Figure 4b shows a selec- ted area diffraction pattern (001) from the same area. In figure 4b, 110 indiumdiffrac- tion spots are observed (weakly) near the missing 200 silicon spots. A dark field micrograph, obtained using these In spots,

is shown in figure 4c in which the cell walls are clearly dellniated. Results pre- sented in figure 4 (a-c) show that the cell walls contain crystalline In, thus demons- trating nucleation of the second phaseafter laser annealing. Similar results have been obtained in the case of Ga and are discus- sed elsewhere /14/. The cells which we

re observed by others subsequent to laser irradiation of silicon crystals with depo- sited metal films /17/.

The cell structure observed in the near surface region after laser annealing is interpreted as resulting from lateral segre- gation of rejected dopant due to constitu- tional supercooling at of the liquid-solid interface /18/. During solidification the concentration gradient of rejected dopant in the liquid leads to a gradient of the freezing temperature of the liquid in front of the interface. If the actual temperature gradient in the liquid

is less than the gradient of the freezing temperature, then a region in front of the interface will be supercooled since the temperature of the

liquid is less than the liquidius tempera- ture. This can lead to interfacial instabi- lity, lateral segregation or rejecteddopant, and the formation of a cell structure.

The condition for constitutional supercooling to exist is /18/

where ATf is the interfacial undercooling in the liquid due to rejected dopant, and D/v is the diffusion length of rejected dopant. If G is less than AT£/(D/V) then constitutional supercooling can lead to interfacial instability and the formation of cell structures. However, if G is grea- ter than ATf/(D/~), the interface will be stable during regrowth. In equation (l),

*Tf depends on the dopant concentration in the liquid and this accounts for the fact that at low doses (i.e. low concentration) a well defined cell structure is notobsemed.

From the phase diagram, and assuming the value for the distribution coefficient k' given in table I, we estimate the inter- facial undercooling ATf for the high dose In case (Figs, 3 and 4) to be %50°C. Using

0

equation ( I ) , with a value for D/v of 150 A, we determine an upper bound for the therraal gradient in the liquid of G 5 3.4

lo7 OC/cm.

Using crystals implanted with different

C4-96

DE

dopants at various doses and usingequation 1 to predict the presence or absence of a cell structure, we can determine both upper and lower bounds for the thermal gradient G. The result /14/ is 4

1 0 6 < ~ 2.6

1 0 ~ ~ ~ / c m . These bounds are in excellent agreement with detailed theoretical calcu- lations /15/ of the gradient in the liquid at the interface ( % 1 0 ~ ~ ~ / c m ) . In order to avoid constitutional supercooling and to incorporate more dopant into substitutional lattice sites, then the thermal gradient in the liquid must be increased. This, in turn, implies that a higher regrowth velo- city must be achieved. Experiments to test these ideas are in progress.

In conclusion we have found that the high velocity of the liquid-solid interfa- ce during laser annealing leads to unique regimes of crystal growth. Group I11 and V dopants can be incorporated into substitu- tional lattice sites at concentrations that far,exceed equilibrium solubility limits.

Values for the (nonequilibrium) interfacial distribution coefficients have been deter- mined and are found to exceed considerably the corresponding equilibrium values. These observations indicate that the forward and reverse reaction rates at the interface during regrowth are considerably different, i,e. that local equilibrium is not maintai- ned. We have determined the maximum subs- titutional solubility which can be achieved for these dopants by our regrowth condi- tions ( v

4.5

lo2 cm/s) . When the total dopant concentration considerably exceeds these maximum values, the non substitutio- nal dopant is found to be highly concentra- ted in the walls of a cell structure inthe near surface region. From the presence or absence of the cell structures wa can esti- mate bounds for the thermal gradient in the liquid at the interface.

Our results can be completely accoun- ted for on the basis of simple melting of the near surface region by the absorbed laser light, followed by liquid phase re- growth from the substrate. There is noneed to invoke the existance of a long-lived

References

/1/ White, C.W., Narayan, J,, and Young, R.T., Science 204 (1979) 461. Also, see reference in Laser-Solid Interac- tions and Laser Processing, ed. by H. J. Leamy and J.M. Poate, (American Institute of Physics, New-York) 1979.

/2/ Wang, J.C., Wood, R.F., and Pronko, P.P., ~ p p l . ~ h y s . Lett. 33 (1978) 455.

/3/ Auston, D.H., Surko, C.I4., Venkatesan, T.N.C., Slusher, R.E., and Colovchenko, J.A., Appl. Phys. Lett. 2 (1970) 437.

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(1979) 788.

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(1979) 312.

/6/ Appleton, B.R., Larson, B.C., Phite, C.W., Wilson, S.R., Narayan, J., and P.P. Pronko, Laser-Solid Interactions and Laser Processing ed. by H. J.

Leamy and J.M. Poate, (American Insti- tute of Physics, New-York) 1979 p. 291.

/7/ Vhite, C.U., Pronko, P.P., Wilson, S.R., Appleton, B.R., Narayan, J., and Young, R.T., J. Appl. Phys. 50 ⁽¹⁹⁷⁹⁾

3261.

/8/ Leamy, H.J., Bean, J.C., Poate, J.R., and Celler, G.K., privatecommunications.

/9/ Baker, J.C., and Cahn, J.W., Acta MettaL 17 (1969) 575.

/lo/ Baeri, P., Campisano, S.J., Foti, G., and Rimini, E., Phys. Rev. Lett. 41

(1978) 1246.

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-

/12/ White, C.W., Wilson, S.R., Appleton, B.R., and Young, F.W., Jr., J. Appl.

phys. 51 (1980) 738.

/13/ Trumbore, F.A., Bell System Tech. J.

39 (1960) 205.

-

/14/ White, C.W., Wilson, S.R., Appleton, B.R., and Narayan, J., Laser and Elec- tron Beam Processing of Pdlaterials,

(in press).

/15/ Wood, R.F., private communications; see also, R.F. Wood, Laser and Electron Beam Processing of &!aterials (in press).

/16/ Kodera, H., Jap. J. Appl. Phys. 2

(1963) 212.

/17/ van Gurp, G.J., Eggermont, G.E.J., Tamminga, Y., Stacy, W.T., and Gijsbers, J.R.I.I., Appl. Phys. Lett.

35 (1979) 273.

-

/18/ See for example, Jackson, I : . A . , in Treatise on solid State Chemistry, Vol. 5, ed. by N.B. Hannay (Plenum Press, New York) 1975 Chapter 5.

/19/ Van Vechten, J.A., these proceedings.

/20/ Hoonhout, D., and Saris, F.W., these proceedings.

plasma as hypothesized by others /19,20/.